Four wheel drive vehicles often have either front or rear wheels that have selectively driven or non-driven modes that operate by selectively engaging or disengaging the wheels from an associated drive train. In these four wheel drive systems, typically, a transfer case, coupled to the vehicle transmission, is designed such that it can be engaged or disengaged from the drive train. The engagement of the transfer case with the drive train selectively supplies power to a set of wheels. Although efficiencies can be obtained by solely disengaging the transfer case when not needed, it is known in the art that disengaging the wheels from the drive train improves driving efficiencies dramatically.
Where a transfer case is selectively disengaged from the drive train and the associated wheel set remains engaged, losses may result from the wheels “back driving” the system and having to overcome the rotational moment of inertia of half shafts, axle carrier assemblies, pinion shafts, and associated frictional losses in bearings, seals and gears. Therefore, it is desirable to have a wheel disconnect system that works in concert with the transfer case, engaging when the transfer case is engaged and disengaging when the transfer case disengages. It should be recognized that such wheel end disconnect systems may be used on either the front or rear wheels, depending upon which wheel set has the primary drive function.
Prior art wheel end disconnect systems include a spindle supported by bearings in a support coupled to the suspension of the vehicle, having an outer end portion adapted to mount a wheel thereon, and an inner extending portion, disposed towards the center of the vehicle, with splines or other engagement elements formed on an outer diameter of the spindle's inner extending portion. Generally, a wheel spindle drive element is mounted on the inner extending portion of the spindle, the spindle drive element having splines or other engagement elements formed on an inner diameter such that the inner diameter engagement elements of the spindle drive element mate or mesh with the engagement elements on the outer diameter of the spindle inner extending portion. Significantly, the engagement elements on the outer diameter of the spindle inner extending portion and the inner diameter of the spindle drive element are formed on each component prior to assembly. The assembly of the spindle drive element onto the spindle requires clearance between the mating members and closely controlled dimensional characteristics and tolerances on the mating members. Any loss in the axial retention of the assembly could allow a shock load under changing torque input due to the clearance between the mating members. A an axial retention mechanism is used to retain the spindle drive element onto the spindle, preventing the spindle drive element from sliding off of the spindle. The retention mechanism may be a number of mechanisms known in the art, including, for example, a nut or a post-assembly outwardly formed flange integral with the spindle. Precise forming of the engagement elements to ensure secure meshing may be difficult, and limits the torque transmission through those elements.
A drive mechanism adapted to rotate under power is mounted adjacent the flange and to the vehicle, with an engaging element or clutch slideably mounted thereon, such that it can be selectively moved into or out of engagement with the spindle drive element, and through the spindle drive element to the wheel. The engaging element or clutch requires an axial force to disengage from the wheel assembly. Generally, a spring is also provided, that urges the engaging clutch back into engagement once axial force has been removed.